Upgrading the ‘Bottom of the Barrel’

The highest yield of straight run product as a percentage on crude is the residue from the atmospheric distillation of the crude feed. In most Middle East crudes this ranges from around 40 vol% for the lighter crude oils to 50% and higher in case of the heavier oils. It can be, and often is, the major factor in crude oil slate selection and in the operation of the refinery itself. In most cases a large portion of the residue can be blended off to meet the fuel oil product in the refinery’s production plan. In other cases, perhaps equally as common, a major portion of the atmospheric residue is further distilled, under vacuum, to distillates which can be processed to meet gasoline and middle distillate product slates or lube oil blending stocks. The residue from this vacuum distillation, now considerably smaller in volume, is routed to fuel oil or to a bitumen product pool.

There are cases howeverwhere the quantities of both atmospheric and vacuum residues are high enough to limit the refinery’s throughput or limit its production of the more valuable products and thus limit the refinery’s profitability. In such cases, the conversion of these residues becomes attractive, and, in some cases absolutely imperative. This latter case refers to those refineries that have no or a very small fuel oil market. The upgrading of these residues is accomplished by the indirect processing of the atmospheric residue, processing the distillates, by catalytic cracking or hydrocracking and then thermally cracking the vacuum residue. Most of the processes involved with the conversion of the vacuum distillates are described in other chapters of this Handbook dealing with hydrocracking and fluid catalytic cracking.